Added a setSize() function to the grid widget

I implemented the Grid::setSize() function following the description written here : http://forum.tgui.eu/index.php/topic,101.msg361.html#msg361

I used the following code to test (a modified version of the login screen), if it can help you:
(As it was just for test it is not really a clean and well-commented code.)
```cpp
#include <TGUI/TGUI.hpp>

void loadWidgets( tgui::Gui& gui )
{
    // Create the background image
    tgui::Picture::Ptr picture(gui);
    picture->load("../xubuntu_bg_aluminium.jpg");
    picture->setSize(800, 600);

    // Create the grid
    tgui::Grid::Ptr grid(gui);
    grid->setSize(800, 600);

    // Create the username label
    tgui::Label::Ptr labelUsername(*grid);
    labelUsername->setText("Username:");

    // Create the password label
    tgui::Label::Ptr labelPassword(*grid);
    labelPassword->setText("Password:");

    // Create the username edit box
    tgui::EditBox::Ptr editBoxUsername(*grid, "Username");
    editBoxUsername->load("../../widgets/Black.conf");
    editBoxUsername->setSize(250, 40);

    // Create the password edit box (we will copy the previously created edit box)
    tgui::EditBox::Ptr editBoxPassword = grid->copy(editBoxUsername, "Password");
    editBoxPassword->setPasswordCharacter('*');

    // Create the login button
    tgui::Button::Ptr button(*grid);
    button->load("../../widgets/Black.conf");
    button->setSize(260, 60);
    button->setText("Login");
    button->bindCallback(tgui::Button::LeftMouseClicked);
    button->setCallbackId(1);

    tgui::Button::Ptr b(*grid);
    b->load("../../widgets/Black.conf");
    b->setText("100x700");
    b->bindCallbackEx([](const tgui::Callback& cb){
                            static_cast<tgui::Grid*>(cb.widget->getParent())->setSize(100,700);
                        },
                      tgui::Button::LeftMouseClicked);

    tgui::Button::Ptr b2(*grid);
    b2->load("../../widgets/Black.conf");
    b2->setText("650x450");
    b2->bindCallbackEx([](const tgui::Callback& cb){
                            static_cast<tgui::Grid*>(cb.widget->getParent())->setSize(650,400);
                        },
                      tgui::Button::LeftMouseClicked);

    tgui::Button::Ptr b3(*grid);
    b3->load("../../widgets/Black.conf");
    b3->setText("800x600");
    b3->bindCallbackEx([](const tgui::Callback& cb){
                            static_cast<tgui::Grid*>(cb.widget->getParent())->setSize(800,600);
                        },
                      tgui::Button::LeftMouseClicked);

    tgui::Button::Ptr b4(*grid);
    b4->load("../../widgets/Black.conf");
    b4->setText("900x1000");
    b4->bindCallbackEx([](const tgui::Callback& cb){
                            static_cast<tgui::Grid*>(cb.widget->getParent())->setSize(900,1000);
                        },
                      tgui::Button::LeftMouseClicked);

    tgui::Button::Ptr b5(*grid);
    b5->load("../../widgets/Black.conf");
    b5->setText("AlignRight");
    b5->setTextSize(10);
    b5->setSize(75, 24);

    grid->addWidget(labelUsername, 0, 0);
    grid->addWidget(editBoxUsername, 0, 1);
    grid->addWidget(labelPassword, 1, 0);
    grid->addWidget(editBoxPassword, 1, 1);
    grid->addWidget(button, 2, 1);

    grid->addWidget(b, 0, 2);
    grid->addWidget(b2, 1, 2);
    grid->addWidget(b3, 2, 2);
    grid->addWidget(b4, 3, 2);

    grid->addWidget(b5, 2, 0);
    grid->changeWidgetLayout(b5, tgui::Grid::Layout::Layouts::Right);
}

int main()
{
    // Create the window
    sf::RenderWindow window(sf::VideoMode(800, 600), "TGUI window");
    tgui::Gui gui(window);

    // Load the font (you should check the return value to make sure that it is loaded)
    gui.setGlobalFont("../../fonts/DejaVuSans.ttf");

    // Load the widgets
    loadWidgets(gui);

    // Main loop
    while (window.isOpen())
    {
        sf::Event event;
        while (window.pollEvent(event))
        {
            if (event.type == sf::Event::Closed)
                window.close();

            // Pass the event to all the widgets
            gui.handleEvent(event);
        }

        // The callback loop
        tgui::Callback callback;
        while (gui.pollCallback(callback))
        {
            // Make sure tha callback comes from the button
            if (callback.id == 1)
            {
                // Get the username and password
                tgui::EditBox::Ptr editBoxUsername = gui.get("Username");
                tgui::EditBox::Ptr editBoxPassword = gui.get("Password");

                sf::String username = editBoxUsername->getText();
                sf::String password = editBoxPassword->getText();

                // Continue here by checking if the username and password are correct ...
            }
        }

        window.clear();

        // Draw all created widgets
        gui.draw();

        window.display();
    }

    return EXIT_SUCCESS;
}
```